Putting chlorine in water treatment plant is essential for the control and suppression of microbial organisms.
However, existing gaseous chlorine is lethal toxicity and safety issues, so existing chlorination facilities are being replaced quickly by the system of electrolyzing the brine, followed by dosing sodium hypochlorite.
In a conventional general diaphragm-type sodium hypochlorite generators, the membrane electrolyzer is composed of the anode chamber 12 and a cathode chamber 14 divided by the cation exchange membrane 16 as shown in FIG. 1 and FIG. 2; the anode chamber 12 has the anode and the cathode chamber 14 has the cathode. And the node chamber 12 and the cathode chamber 14 are connected to the anode storage tank 20 and cathode storage tank 30, respectively.
The cathode chamber 12 has circulating saturated sodium chloride aqueous solution that was introduced into the anode storage tank 20, and the cathode chamber 14 has circulating pure water that has flowed into the cathode storage tank 30. And the electrolytic reaction is taking place inside the membrane electrolyzer when direct current is supplied to the anode and cathode when the anode and cathode liquids flows in and circulates the membrane electrolyzer.
Chlorine (Cl2) is formed through the electrolytic reaction of chlorine ion (Cl−) in the anode, and the sodium ion (Na+) is passed from the anode chamber 12 to the cathode chamber 14 through the cation exchange membrane 16. And hydrogen gas (H2) and hydroxide ion (OH−) are formed by the electrolysis reaction of the water (H2O) in the cathode; hydroxide ions (OH−) combined with sodium ions (Na+) from the anode to form caustic soda (NaOH).2Cl−→Cl2+2e− (anodic reaction)H2O+2e−→½H2+OH− (cathodic reaction)
Thus formed chlorine gas (Cl2) in the anode chamber reacts with caustic soda (NaOH) generated in the cathode chamber to form sodium hypochlorite (NaOCl) in the gas-liquid contact area 40.Cl2+2NaOH→NaOCl+NaCl+H2O (gas-liquid reaction)
However, in such diaphragm-type sodium hypochlorite generator, oxygen (O2) and hydrogen ions (H+) are generated by the side reaction of the electrolytic reaction in the anode and meet with chlorine ions (Cl−) existing in sodium chloride solution to form hydrochloric acid (HCl), resulting in reduced pH. And in the cathode, the caustic soda (NaOH) is generated as stated above and the pH is increased.H2O→2H++½O2+2e−(anodic side reaction)
Accordingly, if the side reactions are reduced and the current efficiency for chlorine is increased, the pH is relatively reduced and the amount of chlorine is increased. However, if the current efficiency for chlorine is increased, the amount of the chlorine gas (Cl2) transferring to the gas-liquid contact area 40 is reduced, resulting in the decreased efficiency because some part of the chlorine is dissolved in the anode water in the form of hypochlorite (HOCl) (for pH=1, Cl2:HOCl=80:20; pH=2, Cl2:HOCl=30:70; pH=3, Cl2:HOCl=10:90).
Thus, in order to increase efficiency of the generation of sodium hypochlorite, the current efficiency for chlorine is needed to be increased and the pH of the anode is needed to be reduced as much as possible.
In addition, in the case of sodium hypochlorite produced by the gas-liquid reaction, the reaction is done by the stoichiometric ratio Cl 2: NaOH=1:2, as shown in the above gas-liquid reaction expression. However, if there is insufficient amount of caustic soda (NaOH), a by-product hypochlorite (ClO3−) is formed by the reoxidation reaction. Therefore, in order to reduce the occurrence of these by-products, stoichiometric material balance must be matched, so caustic soda (NaOH) is required to be re-injected.
As such, the existing process has the disadvantage of the risk of chemical handling and high cost because of the required additional injection of hydrochloric acid (HCl) and caustic soda (NaOH) to balance the material in the gas-liquid reaction and to increase the chlorine generation efficiency of the anode.